Here-to-fore, there has been widely adopted an exhaust gas recirculation (to be referred as EGR hereinafter) apparatus in which a part of exhaust gas in an internal combustion engine is returned to an intake system, and the part of exhaust gas functions as a kind of inert gas to lower a combustion temperature, thereby reducing a discharge amount of NOx.
Due to recent demands for light weight of a vehicle, and improvement of fuel consumption and performance, there have been proposed many internal combustion engines each provided with a turbocharger as a kind of supercharger. Particularly, a diesel engine is better in light of fuel consumption compared with a gasoline engine, however, has a low engine power and cannot meet high-speed rotation. Therefore, it is very beneficial to use a high torque obtained by installing a turbocharger for overcoming these drawbacks.
Since the EGR apparatus uses a differential pressure between an exhaust pressure in an exhaust passage and a negative pressure in an intake passage to return a part of exhaust gas to the intake system, an internal combustion engine mounted with a turbocharger has the following problems. Namely, in a case where the turbocharger has high efficiency, an outlet pressure (negative pressure) of a compressor disposed in the intake passage becomes higher than an inlet pressure (exhaust pressure) of a turbine disposed in the exhaust passage, and therefore, it becomes impossible to return the part of the exhaust gas to the intake system due to the differential pressure. Therefore, as disposed in Japanese Unexamined Patent Publication No. 10-266866, there is proposed the technology in which an intake throttle valve is disposed on the downstream side of a compressor and, at a low load operation, the intake throttle valve is closed to raise up the negative pressure in the intake passage (namely, to lower the intake pressure), thereby improving an EGR rate.
In such a conventional technology, however, although it is possible to increase the differential pressure by the intake throttle valve, an opening degree of the intake throttle valve becomes extremely small depending on engine operation conditions, possibly resulting in problems of such as engine power reduction and emission performance deterioration. Further, since an engine operation region where the differential pressure between the compressor outlet pressure and the turbine inlet pressure can be sufficiently established is narrow, it is very difficult to reduce NOx in the exhaust gas any further.
The present invention, from a viewpoint of the foregoing problems, has an object of providing an exhaust gas recirculation apparatus and an exhaust gas recirculation method for an internal combustion engine wherein a differential pressure between an exhaust pressure in an exhaust passage and a negative pressure in an intake passage is increased, to improve an EGR rate, thereby reducing an NOx discharge amount.